The Head and Oral Cavity

The digestive system of the donkey is primarily designed to obtain energy from a highly fibrous forage diet, consumed steadily throughout the day. A series of anatomical features in the head and oral cavity are directly related to the need to feed on abrasive foodstuffs.

There are anatomical differences between the head and oral cavity of the donkey and those of the horse and pony and it is important to be aware of them.
 

• The curvature of the combined occlusal surfaces of the cheek teeth (CT), which is known as the curve of Spee, is prominent in some donkeys. In these cases teeth should not be modified in an attempt to correct the condition. They should be maintained following the jaw lines and allowing for normal, uniform clinical crown height.

• Donkeys present a greater degree of anisognathia than do horses (5-7% greater at its widest point); the intermaxillary distance is approximately 30% greater than the intermandibular distance. The disproportion of width appears to stem from a narrower, straighter mandible rather than a wider maxilla. This greater degree of anisognathia may be related to the need to have a greater lateral excursion movement, to allow effective chewing of a diet of very rough forage. This anatomical difference is important to consider when deciding whether enamel ‘overgrowths’ are physiological or pathological structures in donkeys.

• In the donkey the rostral and caudal maxillary sinuses may communicate freely because they are incompletely divided by a ventrally located low bony ridge, which does not extend dorsally enough to divide both sinuses.

• Donkeys have a different anatomical location for the opening of the distal nasolacrimal duct. It is small and found variably on the dorsolateral aspect of the nares, often close to the junction with the false nares. This position is potentially useful to avoid blockage with sand or dust in the habitat that the donkey evolved in.

CLINICAL EXAMINATION

The frequency of examination of the head and oral cavity should be considered and may vary according to the stage of life of the donkey.

• 0—5 years: This is the most dynamic period, with the eruption of deciduous and definitive teeth, and the dental exchange process. It is advisable to carry out more frequent prophylactic examinations (every six months).

• 5—15 years: An annual prophylactic examination may be sufficient during this period, unless developmental problems are identified.

• 15—20 years and above: Teeth may start to suffer anatomical changes and dental disease may be secondary to other conditions. Identified dental conditions need continuous treatment and it is advisable to carry out frequent prophylactic examinations (at least every six months).

Observation

Observation is an important part of any clinical investigation and, whenever possible, donkeys should be observed in their normal environment, eating their normal diet. However, a number of donkeys will be presented at mobile clinics and this observation is not always possible.

• Feeding behaviour: check the chewing movement and identify any abnormal sound or abnormal chewing pattern which may indicate restriction of free jaw excursion. This is very important to consider alongside clinical signs such as quidding or drooling.

• Feeders and drinkers: look for signs of obsessive chewing and/or wind sucking, or remnants of poorly chewed food. Donkeys suffering from dental disorders may drop food in the drinkers while drinking, or may deliberately put forage in the water as a way to soften the food.

• Manure: look for undigested grain or, more commonly, increased fibre length.

Extraoral examination

The extraoral examination is performed as in horses and ponies, but it is important to remember that donkeys are less likely to demonstrate pain. Manual nociceptive tests (pressure tests) may cause the demonstration of pain where disorders are present and may therefore enable identification and assessment of pain which may otherwise go unnoticed.

• Cheek pressure is exerted bilaterally and simultaneously on the cheeks at the level of the maxillary CT by pressing the cheek against the entire CT row, starting from the level of the medial canthus of the eye and progressing rostrally over the masseter muscle to the level of the nasal notch. Signs of discomfort (tossing the head) may be the result of pain from sharp CT enamel overgrowths or any other abnormal wear patterns on the vestibular/rostral aspect of the upper CT (e.g. displaced teeth and focal overgrowths).

• Tongue/hyoid pressure is exerted bilaterally on the intermandibular space, pushing the body of tongue dorsally, leading to close contact between the mandibular CT and the lateral aspect of the tongue. Signs of discomfort may be the result of dental trauma at the back of the mouth causing ulcers or lacerations of the tongue, sharp CT enamel overgrowths, or lingually displaced CT.

A positive response to cheek pressure will indicate the need for clinically reasoned reduction of any enamel points. It must also lead to consideration of methods to be used for the intraoral examination; a fully opened mouth with a speculum in place will push the cheeks tightly against the CT. This will cause discomfort and may lead to soft tissue damage in extreme cases.

Intraoral examination

The animal’s general behaviour should be assessed, and a decision made on the need for sedation before commencing the intraoral examination.

The intraoral examination is undertaken as in horses and ponies, and care must be taken to limit periods of time that the mouth is kept open for examination or for treatment.

It is important to flush the oral cavity using a dosing syringe before using the gag.

Unsedated donkeys may object to oral rinsing and reactions may be violent: the presence of water under pressure in the oral cavity stimulates the soft palate to contract, causing a rostral displacement of the palatopharyngeal arch, leading to a blockage of the entrance to the trachea. Animals will calm down and start breathing normally within 10 seconds, but it may be necessary to stimulate the throat region so that the donkey will swallow and relax the adjacent structures. Therefore the mouth should not be flushed with the gag on and the mouth open.

Many donkeys allow intraoral examination and non-painful procedures during treatment without the need to be sedated. When working with unsedated donkeys, keeping the head high may reduce their attempts to push forward. Animals should never be left unattended with a gag in place.

Sedation, if appropriate, should be carried out as in horses and ponies, using the standard doses as stated for equines, but with more frequent top-ups if required.

There are some different considerations that are important to note when examining a donkey rather than a horse and pony.

• In donkeys with absent or a few stable incisors, it may be preferable to use mouth gags with ground-out incisor plates rather than traditional ‘gum’ bars to avoid trauma to the interdental space, which may occur even when they are padded. Dental impression material may be used to line the incisor plates of mouth gags used in donkeys with a complete loss of incisors, and this tailor-made liner may be reused at subsequent visits.

• It is suggested that ventral curvature of the occlusal aspect/surface of the incisors is a normal feature in donkeys. However, other studies suggest it may be an acquired disorder, increasing significantly with age and possibly due to disorders in the CT that affect the normal movement of the jaw. If indicated, phased correction is advised, restoring normal incisor conformation, but care must be taken to avoid loss of incisor occlusion.

• In donkeys it is common to find up to 4 wolf teeth (one per arcade). In normal cases, often these teeth do not contact or interfere with a correctly used bit, but this may be assessed during the intraoral examination and extractions advised on a case by case basis.

Ancillary diagnostic tests

These tests are similar to those used in horses and ponies, with radiography being the most commonly used diagnostic method. Rhinoscopy may be useful but it is important to remember that the nasal passages of the donkey are smaller than similar sized horses and ponies, so good lubrication and use of the correct diameter endoscope are important.

ORAL AND DENTAL DISORDERS

There is limited, robust published information on donkey oral health and dental disease. However, those studies that are available indicate that a wide range of pathology is seen in a significant proportion of animals in all the environments in which they are kept. The pathology often increases in incidence and severity with age and may be more prevalent in certain populations or breeds. Individual animals often present with multiple disorders.

Developmental disorders of the teeth

Developmental disorders of teeth in donkeys are similar to those found in horses and ponies.

Those disorders with donkey specific differences include:

Craniofacial abnormalities which need early detection and treatment to prevent development into more severe conditions. These may have a genetic basis and breeding from affected animals is not recommended.
 

Dental dysplasia is relatively uncommon.

Polyodontia is relatively uncommon but very important. Fully erupted supernumerary teeth are reported with an increasing prevalence in elderly donkeys. This suggests a late-onset eruption process. Supernumerary teeth are commonly located in the caudal aspects of the maxillary CT in the donkey and are therefore easily missed. 

Retained deciduous teeth will mainly affect the donkey over 2.5 years of age. A knowledge of eruption times of the teeth for donkeys is important to avoid premature removal of deciduous teeth, which may affect the underlying developing permanent tooth. Recent studies show that teeth fragments are more commonly found on the medial aspect of the maxillary CT rows in donkeys, so extra care is needed during the extraction of these fragments to avoid laceration of the great palatine artery. The most common deciduous teeth to be retained are central incisors (01s) and deciduous 08s.
 

Eruption cysts are relatively common and a normal, transient developmental feature.

Abnormal dental eruption, displacements and developmental diastemata are all found in donkeys. A high prevalence of 03s distally displaced has been described.
These abnormalities can increase the likelihood of complicated fractures in the displaced teeth and food entrapment, with associated gingivitis and periodontitis.
 

Acquired disorders of the teeth

Those disorders with donkey specific differences include:

Hypodontia (loss of teeth) is seen in both incisors and CT and is common in donkeys and particularly in the geriatric. This may be due to periodontal disease.

Abnormalities of wear increase with age and affect the occlusal surface of both incisors and CT. It is the most common disorder of CT; mainly seen as enamel overgrowths affecting all CT.

Ventral curvature of the incisors which may be an age-related acquired disorder. Studies report a 10% prevalence in donkeys less than 10 years old and up to 80% in animals older than 25 years (Rodrigues et al, 2013a). It is probably secondary to CT disorders interfering with the correct chewing movement. If indicated, phased correction is advised, balancing and correcting the disorders of CT, while restoring the normal occlusal incisor conformation.
 

Focal overgrowths affecting the 06s and the 11s; mainly found in those donkeys with some degree of incisor malocclusion.

Overgrowth of the complete tooth due to poor or absent occlusal contact is not uncommon and frequently bilateral. These overgrown teeth interfere with normal chewing movement and may predispose to the development of wave mouth or shear mouth.

Studies in working equids suggest a high prevalence of CT enamel overgrowth and the majority of associated soft tissue lesions are located on the cheeks beside the maxillary 06s and 07s. This corresponds with areas where halters may exert pressure and may indicate a different aetiology for this disorder than in companion donkeys.

It may be that higher protruding enamel on the CT is physiological and a normal feature for donkeys, in which case the pain response to the nociceptive tests (cheek and tongue/hyoid pressure) may be used as an indicator for the need for correction of enamel overgrowths.

Early detection and treatment of abnormalities of wear can avoid its development into more severe conditions.

Dental displacements are recorded. Molar displacements, with diastema formation and the potential for soft tissue trauma are seen. If not treated, these displacements may be severe and, if periodontal disease is not advanced, these teeth may have very solid attachments.

Diastema with food entrapment and associated periodontal disease is a very painful disorder. Acquired diastema is a common disorder of the donkey, affecting mainly mandibular CT. They are an age related disorder, increasing in extent and prevalence with age.

A thorough visual and digital oral examination is essential to investigate clinical signs such as; excessive salivation, oral malodour, inappetence/anorexia, colic (especially impaction), hyperlipaemia, and weight/condition loss.
 

Periodontal disease (PD) is a common disorder, as in horses and ponies.

Periodontal disease in donkey cheek teeth demonstrates an age related increase in severity, simultaneous to an higher number of affected teeth compared to horses.

Periodontal disease in donkey incisor teeth demonstrates no age related increase in severity, but an increase in the number of incisors affected.

Calculus was recorded as the main predisposing factor for incisor PD. It may be reversible if the calculus is removed at an early stage.

More severe cases can lead to attachment loss and periodontitis.

Diastemata and retained deciduous incisors are related to PD in the definitive incisors. In CT almost 90% of teeth diagnosed with PD are caused by diastemata.

Other less common causes of PD include peripheral caries, damage to the normal tight junction between gingiva and peripheral cementum at the gingival margin, leading to attachment loss in some cases, and retained deciduous CT.

Caries may affect all dental components at a peripheral and infundibular level. This disorder has been shown to increase with age. Prevalence and severity of the degree observed, and the causes, are similar to those seen in horses and ponies.

Calculus is often related to the presence of other dental conditions as in horses and ponies. Elderly donkeys may present with considerable malodorous deposits at the canines and incisors, but also around the maxillary premolars adjacent to the ostium of the parotid duct.

There appears to be less sexual dimorphism relating to the canines than demonstrated in horses and ponies, and both male and female donkeys may be affected.

Fractures are reported to have a variable incidence. The majority of fractures in incisors are uncomplicated, affecting only the external components of the teeth.

Due to the anatomy of CT, all fractures (even small slab fractures) have the potential for pulpar exposure (i.e. are complicated fractures), so careful clinical inspection of these areas is important, using dental probes and a mirror.

In one study some donkeys were observed with small fractures in the rostral aspect of 06s that appeared to be the site of former small focal overgrowths that fractured under the high pressures of normal equine mastication. (Rodrigues et al, 2013b) The pulp horn inevitably involved appeared to be sealed off.
 

Apical infection presents in donkeys as in horses and ponies. 

Soft tissues lesions

Lesions may be seen in the soft tissues and are common in working donkeys. Inappropriate and poorly fitting bits or restraint systems with tight nosebands are some examples where soft tissues can be compressed against the teeth, causing lesions.
 

TREATMENT

Treatment follows established best practice in horses and ponies.

A treatment plan is essential when treating significant dental disease. The plan should consider staging procedures in order to avoid post-treatment stress, anorexia and potential hyperlipaemia.

A full clinical examination, preferably including haematology and biochemistry screens, is strongly recommended to inform a treatment plan, particularly in elderly or sick donkeys or those with potentially reduced appetite.

It is preferable to perform staged reductions over time rather than one major corrective procedure in all but the most minor overgrowths.

Care must be taken to avoid direct or indirect pulp exposure during dental reductions. Whilst working on the occlusal surfaces, the staining of the secondary dentine may lighten in colour but must remain visible at all regions of the crown.

As donkeys typically live longer than horses, it is possible to perform minor reductions and regain occlusion in animals in their late teens/early twenties. It is, however, occasionally best for the elderly patient to allow minor focal overgrowths to remain in order to allow as much occlusion as possible.

It is not uncommon in elderly donkeys, particularly on the maxillary arcades, that once one tooth is extracted, two or three others lose their stability and also require extraction. Radiographs may be helpful to identify such cases prior to treatment.

Appropriate analgesia should be considered and prescribed.

Donkeys with dental disease or undergoing dental treatment will frequently require dietary modification.

PREVENTION AND QUALITY OF LIFE

Dentistry should be prophylactic. A regular programme of routine examinations and care will allow the early diagnosis and treatment of oral and dental disorders and avoid progression into potentially significant pathology.

• Good dental care has proven benefits for the health and welfare of donkeys.
• All donkeys require a regular dental care programme that includes full oral examination, appropriately adapted to the individual and their age.
• A full clinical examination and assessment should be undertaken before treating significant dental disorders, particularly in elderly donkeys.
• Dental disease is an integral component of any quality of life assessment.
• Diets should be modified appropriately to avoid problems of colic, choke (oesophageal obstruction) and weight loss.

Consideration must be given to the quality of life of the donkey when considering any treatment plan. Important questions are:

• what is the overall aim of treatment and what procedures are required?
• can they be competently undertaken with or without hospitalisation?
• what is the likely recovery period and what will post-procedural function be?
• how will pain and diet be managed?
• what other issues does the donkey present with?

Many clinical situations may have a direct impact on the quality of life of donkeys, with some being life threatening, as described for cases of extreme craniofacial abnormalities in new-born donkeys.

In older animals, CT loss, dental functional capacity and average faecal fibre length provide useful information for quality of life assessment, treatment planning and post-procedural care/management considerations, in addition to dealing with owner expectations.

The location of missing CT and the loss of multiple adjacent teeth have a bearing on donkey longevity.